Closed-form analysis of intra/inter-band cross-modulation in multiband radio-over-fiber systems

A theoretical and experimental investigation of the nonlinear behaviors of intra- and inter-band cross-modulation (XM) in a multiband radio-over-fiber (RoF) system is presented for the first time. A closed-form theoretical analysis is proposed to characterize and validate the data XM induced by the nonlinearity of optical Mach-Zehnder modulator (MZM). Nonlinear behaviors of the intra-band XM between the in-phase and quadrature (I/Q) components of a single-band vector signal, as well as the inter-band XM between multiband combinations of scalar and vector signals are investigated, and consistent agreement has been verified between the theory and experiments. It was confirmed that scalar signals only suffer from inter-band XM; whereas vector signals suffer from both intra- and inter-band XMs. Intra-band XM induces constellation compression to vector signals; while inter-band XM induces eye diagram degradation to scalar signals, as well as nonlinear constellation scaling to vector signals. Moreover, QPSK shows superior tolerance to MZM nonlinearity than higher order QAM formats due to it intrinsic resistance to intra-band XM.